M-type microwave signal delay tube

ABSTRACT

A microwave signal delay tube is disclosed of the so-called &#39;&#39;&#39;&#39;Mtype&#39;&#39;&#39;&#39; wherein the modulated electron beam sequentially travels around an inner and outer concentric, circular drift section in opposite directions. The electron beam reversal which takes place after the beam has traveled around one of these sections has the effect of averaging the velocity slip in the beam, brought about by the circular drift geometry of the tube.

United States Patent 11 1 Belohoubek 14 1 Jan. 28, 1975 1 M-TYPE MICROWAVE SIGNAL DELAY TUBE [75] Inventor: Erwin F. Belohoubek, Kendall Park,

22 Filed: Mar. 6, 1968 211 App]. No.: 711,469

3,153,742 10/1964 Kluver 315/393 Primary Examiner-Maynard R. Wilbur Assistant Examiner-D. M Potenza Attorney, Agent, or Firm-R. S. Sciascia; L. l. Shrago [5 7] ABSTRACT A microwave signal delay tube is disclosed of the socalled M-type wherein the modulated electron beam sequentially travels around an inner and outer [52] 11.5. CI 315/393, 315/3951 concentric, circular drift i n in opposite direc- [51] Int. Cl. H01j25/34 time h c r n eam r er al hich takes place [58] Field of Search 315/35, 3.6.39, 39.3, after h beam h r l r und one of these sec- 315/39.5l tions has the effect of averaging the velocity slip in the beam, brought about by the circular drift geometry of 1 References Cited the tube.

A PATE UNITED ST T NTS 9 Claims, 2 Drawing Figures 2,812,473 11/1957 Mourier SIS/39.3

OUT PUT 21 22 2o 5 o 3 /30 Q P 3 Q 3 INPUT 9 19 w u M-TYPE MICROWAVE SIGNAL DELAY TUBE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates generally to signal delay devices and, more particularly, to an adjustable microwave signal delay tube employing an electron beam traveling through crossed electric and magnetic fields.

In the past, it has been proposed to utilize traveling wave tubes as variable signal delay devices. In one type of traveling wave tube, the so-called M-type, the electron beam moves through crossed electric and magnetic fields. The microwave signal which is to be subjected to the delay is transferred to the beam at an input coupler and, thereafter, the modulated beam proceeds at a relatively slow velocity which can be adjusted through a drift section. After traversing this section, the modulated beam passes through an output coupler. Here, the microwave signal is extracted and, thereafter, the beam terminates at a remote collector electrode.

The total microwave signal delay of such a tube as the one just described corresponds to the time it takes for the electrons to travel from the beginning of the input coupler to the end of the output coupler. This delay, therefore, is determined primarily by the translational velocity of the electrons as they move through the drift region of the tube. Since this velocity is governed by the ratio of the DC electric to magnetic field intensities, a variable signal delay may be achieved by simply changing the magnitude of the electric field present in the drift space.

One of the conditions limiting the maximum signal delay achievable is the signal loss brought about by velocity slip between different sections of the electron beam. This slip, which produces a loss in the microwave signal and a deterioration of its wave form, is caused by variations in the electric or magnetic field over the beam cross section. Typical examples are space charge potential depression, mechanical differences in the spacing of the drift plates, nonuniformities in the magnetic field, DC electric side focusing fields, etc. In each of these instances, either the electric or magnetic field in one section of the beam is slightly different from that in another. Consequently, different electron velocities occur within the beam.

In one type of delay tube, the drift space has a circular configuration..This geometry is advantageous since it provides -a maximum signal delay for a tube of given outer dimensions. However, this construction introduces a certain amount of slip in the electron beam since those electrons which are at the greatest radial distance from the center of the tube travel longer distances through the drift space than those electrons, for example, which are at the shortest radial distance from this same point. For slip-free flow, the angular velocity, and not the absolute velocity, of these electrons must be constant.

In the-present invention, this slip is reduced by having the electron beam pass around a circumferential portion of the tube, first in one direction and then back in an opposite direction. The folding of the beam, it will be appreciated, reverses the relative positions of the electrons so that, for example, those which are at the greatest radial distance from the center on the outgoing passage are now nearest the center on the return passage, and vice versa. Consequently, those electrons which have the largest path when the electron beam is moving in one direction through the drift space have the shortest path on the return trip. Thus, the velocity slip normally caused by the circular geometry is averaged. Folding of the beam is possible since the electrons follow equi-potential guidelines and no mixing between the electrons takes place as they travel each leg of the above path. The present delay tube thus has a long drift length and still is of a compact size.

In applicants copending application, Ser. No. 572,153, filed Aug. 12, 1966, now U.S. Pat. No. 3,639,802, there is disclosed the concept of similarly curving the drift electrodes in order to focus the electron beam. In the present invention, this radius of curvature is made relatively large and is not related to the curvature of the drift space. This large drift electrode radius has the advantage of permitting the beam to spread in width. Thus, the space-charged density in the beam is reduced and the slip caused by this condition minimized.

It is the primary object of the present invention to provide an improved, adjustable, circular microwave delay tube of the M-type wherein velocity slip in the beam is reduced.

A second object of the present invention is to provide a traveling wave delay device wherein the electron beam is folded so as to pass sequentially around the circumference of the tube in opposite directions.

Another object of the present invention is to provide a microwave delay tube of the M-type having a considerable increase in the delay-bandwidth product without change in the weight or outer dimensions of the tube.

A still further object of the present invention is to provide an M-type, circular microwave delay tube wherein the modulated electron beam is turned approximately Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a delay tube embodying the present invention; and

FIG. 2 is a cross-sectional view of the apparatus of FIG. 1, taken through line 2-2 illustrating the inner and outer drift spaces.

Referring now to H6. 1 of the drawings, it will be seen that the signal delay tube in one contemplated embodiment comprises an inner ring member 1 and an outer, concentrically spaced ring member 2, both fabricated from nonmagnetic material so as to avoid magnetic field distortions in the annular space therebetween. Inner ring member 1 supports an electron gun assembly 3 which forms and launches a strip-type electron beam. This gun, which includes a conventional cathode element, a beam forming electrode and an anode, all supported in a unitary manner by suitable alumina rods, is operated with a low anode voltage so as to launch a low velocity beam. This mode of operation minimizes the guideline spread of the beam.

Since the delay tube utilizes a fast wave coupler 4 as the microwave signal injection means, an adiabatic transition 5 is included between the gun and the coupler. This transition, which brings the beam up to an electron velocity sufficient for use with the fast wave, is formed by a tapering finger 6 secured to the outer ring member 2 and a confronting portion of the inner ring member 1. The intervening space of decreasing separation produces a transverse electrical field of increasing intensity in the direction of beam travel when a suitable voltage is applied thereacross.

The microwave signal which is to be delayed is coupled to fast wave coupler 4 in a conventional manner. This coupler, as is well known, is a capacitive loaded coaxial resonator with the capacitive area acting as the modulating gap for the electron beam. The cavity resonator of the coupler is formed by a straight opening machined into inner ring member 1. A coaxial line 7 is inserted through an appropriate aperture in the wall of this ring, and its inner conductor terminates at a conducting disk which acts as the above capacitive element.

Before the modulated electron beam enters the first circular drift section of the tube, it must be slowed down, and this is accomplished by a second adiabatic transition 8 whose operation is similar to the one previously described except, here, the confronting wall surfaces of the finger 6 and ring member 1 provide an interaction area of increasing separation and diminishing electrical field intensity in the direction of beam travel.

The drift space of the present tube consists of two circular sections, an inner one 9 and an outer one 10, both interconnected by a turn-around portion located back near the beam forming apparatus. The two drift sections are formed by the circular drift electrode 11 which partitions the space between the ring members 1 and 2 into two equal areas.

As perhaps best seen in FIG. 2, which is a crosssection through a portion of the delay tube, the circular drift electrode 11 is maintained midway between the two ring members by nonconducting supports 12 and 13. These supports are secured to the side closure plates 14 and 15 which seal off the annular space of the tube and permit its evacuation by conventional means. In an alternative construction, a pair of supporting rings may be employed to insure the precise positioning of the circular drift electrode over its complete length.

The modulated electron beam, after it has been slowed down by transition 8, enters the lower drift space 9 and proceeds clockwise around this space until it gets to the remote end of drift electrode 11. Here it enters a turn-around space, is turned 180, and enters the upper drift space 10. The turning of the beam is accomplished by a pair of electrodes, an inner electrode 16 of semi-circular shape supported from the remote end of drift electrode 11 and an outer electrode 17 which is formed with an arcuated surface. Inner electrode 16 is maintained at a positive potential which is higher by AV than the positive potential applied to the drift electrode 11. The outer electrode 17 is DC isolated from the conducting films attached to ring members l and 2 and is kept at a potential -AV below that of these conducting films. These potentials produce a local acceleration of the electron beam at the turning point. The increased electric field also increases the beam side-focusing force at this point and prevents the beam from spreading.

As mentioned hereinbefore, folding or reversing the direction of the beam results in a transposition of the electrons which are at the outer and inner limits of the original beam. Consequently, those electrons which are in the outer position of the outgoing beam and travel the greatest distance through drift space 9 are now in the inner position of the returning beam and travel the shortest distance through drift space 10. This compensation thus minimizes the amount of slip produced by the circular geometry of the tube. The most effective compensation is realized when the radius of curvature of the circular drift electrode r is much larger than 3. the distance between the centers of the outgoing and returning beams. As mentioned hereinbefore, the turning of the beam is effective because no mixing between electrons takes place, that is, those electrons which are at the outer limit of the outgoing beam end up at the inner limit of the returning beam, and vice versa. The reason for this is that the electrons follow equipotential guidelines.

After the beam completes its turn, it proceeds counterclockwise around outer drift space 10 until it comes to a third adiabatic transition 20, located before a fast wave coupler 21 which serves as the microwave signal extraction means. Transition 20 is similar to the other transitions previously mentioned, and it operates to increase the velocity of the beam so that a fast wave type of coupler may again be used in the tube. The electron beam, after it passes through coupler 21, terminates at a collector electrode 22 which may be an isolated area of metallic film on the top surface of finger 6 and electrically insulated therefrom. It will thus be seen that the present tube has a relatively long over-all drift space, yet requires a magnet of only moderate size because of the arcuated nature of this drift space.

As seen in FIG. 2, the confronting surfaces of ring members 1 and 2 are similarly curved over their complete circumferential lengths. These surfaces have suitable electrically insulated coatings applied thereto, and metallic films 23 and 24 are evaporated over these coatings to form the outer drift electrodes. Alternatively, the confronting surfaces of the ring members may be flat, and the metallic films, themselves, curved to form the concave electrodes. Inner drift electrode 11 may be a solid conducting member whose upper and lower boundary surfaces are also curved in a convex manner. However, the outer drift electrodes and the inner drift electrode should have the same radius of curvature R. This radius of curvature in the present invention is deliberately made large compared to the radius of curvature r,, of the circular drift electrode 11, which may be thought of as the average radius of curvature of the drift spaces of the tube. By using a large radius of curvature for the electrodes, the velocity slip caused by the l/R variation of the electric field is kept small. This large value of R also has the advantage of allowing the electron beam to spread in width. This spreading reduces the space charge density in the beam and further reduces velocity slip caused by space charge depression. Also, since the folded drift space geometry substantially doubles the drift path of a delay tube of fixed outer dimensions, the tube may operate at higher beam velocities. Such a mode of operation further reduces the space charge density of the beam and the slip caused thereby.

In the operation of the delay tube, an external magnetic field is present acting at right angles to the electric field throughout the complete drift space. In FIG. 1 the direction of this field is into the paper as shown by the inscribed cross. A positive voltage +V is applied to the circular drift electrode 11, and a negative voltage V,, is applied to the metallic films 23 and 24 on the inner surfaces of the ring members. Suitable DC feedthroughs, such as 30 and 31, may be formed in the ring members to permit electrical connections to the appropriate surfaces for excitation of the electrical fields.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. Signal delay apparatus comprising, in combination,

means for forming an electron beam;

means for modulating said electron beam with a signal which is to be delayed;

means for directing said modulated electron beam at a controllable velocity over a folded drift path; and means for thereafter extracting said signal from said modulated beam.

2. Signal delay apparatus comprising, in combination,

means for forming an electron beam;

means for modulating said electron beam with a signal which is to be delayed; means for directing said modulated beam at a controllable velocity over a first arcuated path in one direction and then over a second arcuated path in an opposite direction whereby the electrons in the inner and outer limits of the beam travel substantially equal distances and the velocity slip due to the curvature of these paths is reduced; and

means for thereafter extracting said signal from said modulated beam.

3. In an arrangement as defined in claim 2 wherein said first and second arcuated paths are concentric.

4. ln an arrangement as defined in claim 3 wherein the radius of curvature of both arcuated paths is large compared to the distance between the center lines of said arcuated paths.

5. In an arrangement as defined in claim 2 wherein said modulated electron beam is turned approximately after it travels over said first arcuated path and before it proceeds over said second arcuated path.

6. In an arrangement as defined in claim 5 wherein the velocity of said modulated electron beam is increased as it is turned through said 180 whereby any spreading of the modulated electron beam during this turn is minimized.

7. A signal delay apparatus comprising, in combination,

means for forming an electron beam;

means for modulating said electron beam with a microwave signal;

means for reducing the velocity of said modulated electron beam;

means for thereafter guiding said modulated electron beam through a drift space which includes a first arcuated section, a turning section wherein the direction of the beam is changed and a second arcuated section which is concentric with said first arcuated section;

means for increasing the velocity of said modulated electron beam after it has passed through said drift space; and

means for thereafter extracting said microwave signal from said modulated beam.

8. In an arrangement as defined in claim 7 wherein said means for guiding said modulated electron beam through a drift space includes a pair of concentrically disposed ring members having an annular space therebetween and an arcuated electrode positioned between the confronting surfaces of said ring members and subdividing said annular space into an upper and lower drift section.

9. In an arrangement as defined in claim 8 wherein the inner surfaces of said ring members have a concave curvature and the outer surfaces of said electrode have a convex curvature with both radii of curvature being the same. 

1. Signal delay apparatus comprising, in combination, means for forming an electron beam; means for modulating said electron beam with a signal which is to be delayed; means for directing said modulated electron beam at a controllable velocity over a folded drift path; and means for thereafter extracting said signal from said modulated beam.
 2. Signal delay apparatus comprising, in combination, means for forming aN electron beam; means for modulating said electron beam with a signal which is to be delayed; means for directing said modulated beam at a controllable velocity over a first arcuated path in one direction and then over a second arcuated path in an opposite direction whereby the electrons in the inner and outer limits of the beam travel substantially equal distances and the velocity slip due to the curvature of these paths is reduced; and means for thereafter extracting said signal from said modulated beam.
 3. In an arrangement as defined in claim 2 wherein said first and second arcuated paths are concentric.
 4. In an arrangement as defined in claim 3 wherein the radius of curvature of both arcuated paths is large compared to the distance between the center lines of said arcuated paths.
 5. In an arrangement as defined in claim 2 wherein said modulated electron beam is turned approximately 180* after it travels over said first arcuated path and before it proceeds over said second arcuated path.
 6. In an arrangement as defined in claim 5 wherein the velocity of said modulated electron beam is increased as it is turned through said 180* whereby any spreading of the modulated electron beam during this turn is minimized.
 7. A signal delay apparatus comprising, in combination, means for forming an electron beam; means for modulating said electron beam with a microwave signal; means for reducing the velocity of said modulated electron beam; means for thereafter guiding said modulated electron beam through a drift space which includes a first arcuated section, a turning section wherein the direction of the beam is changed and a second arcuated section which is concentric with said first arcuated section; means for increasing the velocity of said modulated electron beam after it has passed through said drift space; and means for thereafter extracting said microwave signal from said modulated beam.
 8. In an arrangement as defined in claim 7 wherein said means for guiding said modulated electron beam through a drift space includes a pair of concentrically disposed ring members having an annular space therebetween and an arcuated electrode positioned between the confronting surfaces of said ring members and subdividing said annular space into an upper and lower drift section.
 9. In an arrangement as defined in claim 8 wherein the inner surfaces of said ring members have a concave curvature and the outer surfaces of said electrode have a convex curvature with both radii of curvature being the same. 